The present invention relates to a digital-to-analog converter (hereinafter referred to as a "D/A converter), and more particularly to a D/A converter including an array of capacitors and voltage dividers composed of resistors.
D/A converters including an array of capacitors have in recent years found growing use as output means in digital electronic devices since they can easily be fabricated as MOS ICs.
A number of weighted capacitors used as a capacitor array are generally formed by MOS capacitors and are of a greater dimensional accuracy than other passive components, so that D/A converters of the above type have highly accurate characteristics.
However, a D/A converter with a capacitor array uses several hundred MOS capacitors or more of a unit capacitance (for example, 0.1 pF), and hence it is difficult to reduce the IC chip area. To avoid this drawback, it has been practice to add a voltage divider composed of resistors for reducing the number of arrayed capacitors used. Such a technique is disclosed in Japanese Patent Application No. 56-8946 entitled "Digital-to-analog converter", for example.
The capacitor-array D/A converter including a resistive voltage divider and having a resolution of 14 bits or more has a disadvantage in that when actuating capacitor switches, the conversion characteristic is shifted .DELTA.Va or .DELTA.Vb in a positive or negative direction as shown in FIG. 1 of the accompanying drawings, and hence has poor linearity.
When the 14-bit D/A converter is operated under two reference voltages of +3 V and -3 V, for example, the resolution .DELTA.Vc is about 0.4 mV. Where the switches are actuated with a digital input code value 00000000011111, the shifted voltage .DELTA.Va becomes about 11 mV which indicates a variation greater than the resolution voltage.
Such a phenomenon results from variations in the capacitances of MOS capacitors fabricated in an IC and the resistances of the resistors. It has therefore been guite difficult to put to practical use capacitor-array D/A converters having a resolution of 14 bits or greater.